Th1 polarized CD4 T cell responses are important for containment of Mycobacterium tuberculosis (Mtb) infection, but otherwise the features of host protective CD4 T cell responses during Mtb infection are poorly understood. This lack of robust correlates of protection is a major barrier to developing new vaccine and therapeutic strategies. In this project we have analyzed the ability of different subsets of Mtb-specific CD4 T cells to migrate into the parenchyma of the lung tissue when the infection resides. This was accomplished by employing an intravascular staining technique that allows us to discriminate between tissue parenchymal and blood vascular localized T cells by flow cytometry. We show that the pulmonary effector T cell response against Mtb is fundamentally composed of two major subpopulations that preferentially localize to either the lung tissue parenchyma or lung blood vasculature, and we have extensively phenotypically and functionally characterized these two subsets. The frequency of Ag-specific CD4 and CD8 T cells in the lung blood vasculature was greatly elevated compared the peripheral blood and comparable or even higher than the tissue resident parenchymal cells. While the parenchymal T cells displayed elevated levels of multiple activation markers, the intravascular T cells expressed high levels of markers indicative of a highly differentiated phenotype. Interestingly, the intravascular Mtb-specific CD4 T cells displayed greater potential for cytokine production as well as active production of IFNg in vivo compared to their tissue parenchymal counterparts. Most importantly, CD4 T cells with the ability to migrate into the tissue parenchyma were highly protective against Mtb infection, while CD4 T cells that were high producers of IFNg but poorly migrate into the lung displayed minimal control of the infection. Therefore, we have found that not all Mtb-specific effector CD4 T cells are capable of entering controlling the infection in the lung, and identified the subset of Mtb-specific effector CD4 T cells with greatly enhanced migratory capacity that that mediate efficient control of Mtb replication in vivo, indicating that migratory capacity is a critical property of Mtb-specific effector CD4 T cells that may serve as a useful correlate of host protection in tuberculosis. PD-1 is an inhibitory receptor expressed on the surface of activated T cells and we have previously shown that PD-1 dependent inhibition is required for limiting lethal CD4 T cell mediated immunopathology during Mtb infection. Here, we use the intravascular staining technique to examine effector CD4 T cells responses in PD-1 deficient mice after Mtb infection. We observed a marked increase of Mtb antigen-specific CD4 T cells in the lung parenchyma of PD-1 deficient mice compared to WT mice. Moreover, we find that PD-1 deficiency in Mtb infection is associated with a dramatic increase in IFNg production by parenchymal cells. Given our results showing that parenchyma homing CD4 T cells display the highest host-protective capacity in WT mice, these results highlight the importance of proper regulation in limiting immunopathology mediated by effector T cells normally beneficial to the host. Currently, we are investigating the hypothesis that the overproduction of IFN-g directly leads to the immunopathology observed in the PD-1 KO mice. To better understand the role of PD-1 in other settings of chronic infection, we examined its role in regulating CD4 T cell polarization during infection with the opportunistic fungal pathogen Cryptococcus neoformans. Using mulitcolor flow cytometry to characterize the polyfunctional capacity of CD4 T cells in the lungs of C. neoformans infected mice, we find that pulmonary exposure to C. neoformans elicits predominantly a Th2 response, and smaller but observable Th1, and Th17 responses. Previous work has found that Th2 responses are associated with poor outcome and Th1 and Th17 responses with host protection. We find that PD-1 is expressed on each of these effector T cell subsets, with the highest levels on Th2 cells that make IL-5 and IL-13. In PD-1 deficient mice, CD4 T cells making IL-13, IL-5, and IL-4 were decreased while the frequency of IFN-g-producing and T-bet expressing CD4 T cells were increased, and Th17 responses were unchanged. Consistent with the decrease in Th2 responses, PD-1 deficient mice show a decrease in eosinophils in the lung, and in fungal burden. We are currently exploring the mechanisms by which PD-1 promotes Th2 responses and inhibits Th1 in the context of cryptococcal infection. Our previous work on our murine model of Immune Reconstitution Inflammatory Syndrome (IRIS) has identified several major pathways that have a role in the immunopathology following CD4 T cell transfer into M. avium chronically infected T cell deficient mice, including IFN-g and TNF. We have extended these findings now to show that IL-6 is a major mediator of IRIS pathology. We has shown that IL-6 levels spike at the time of immunopathology in M. avium complex/HIV co-infected individuals who develop IRIS after ART. Moreover, we have used the experimental animal model of M. avium associated IRIS to investigate the role of IL-6 in IRIS pathology. Treatment of mice with IL-6 blocking mAbs at the time of CD4 T cell reconstitution or after the onset of disease symptoms resulted in significant reduction in weight loss and prolonged survival. The combination of IL-6 and IFN-g blockade dramatically reduced the signs of illness during experimental IRIS. Therefore, much of the pathology in IRIS results from three of cytokines that are normally associated with host-resistance